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Semiconductor nanowires: from fundamental physics to device applications

Periodic Report Summary 1 - NANOWIRING (Semiconductor nanowires: from fundamental physics to device applications)

Nanotechnology based on semiconductor nanowires promises a new generation of devices benefiting from large surface to volume ratios, small active volumes, quantum confinement effects and integration in complex architectures on the nanoscale. The main issues that the joint research programme intends to address within the project are the following: semiconductor nanowires for sensing, for optoelectronics, for nanoelectronics and for energy harvesting applications. The main objective of the proposed network is to embed a pool of postgraduates and young researchers in a multidisciplinary framework of research and development activities in the emerging field of science and applications based on the unique properties and opportunities offered by semiconductor nanowires.

For sensing purposes the research work is focussed towards the realisation of prototype device systems driven by the industrial need proposed by FIAT, but also motivated by the increasing demand of biocompatible, cheap and accurate bio-sensors. These include ZnO and SiC nanowire-based (i) gas sensor prototypes with high reproducibility, sensitivity and good stability; (ii) piezoelectric sensors; (iii) bio-sensors. Of concern to the latter issue the stable biofunctionalization of ZnO nanowires for DNA sensing applications using the organosilane GOPS has been achieved. The bifunctional linker GOPS was used to immobilize a monolayer of DNA capture molecules on the ZnO nanowires. The successful functionalization could be verified with fluorescence microscopy. The functionalized wires may be used as building blocks for electrically driven DNA target molecule detection. In fact the current through an intrinsically n-type ZnO nanowire should decrease as the negatively charged DNA target molecules induce a depletion zone inside the nanowire [Niepelt et al. Nanoscale Research Letters 6:511 (2011)]. Theoretical modelling has been applied to consider the possibility of forming stable hybrid interfaces with different aromatic rings as bricks on SiC surfaces for biological sensor applications. It was demonstrated that only pyrrole can form covalent bonds to the clean substrate, with strong modifications of its electronic properties around the Fermi level. The results rule out the possibility of forming stable hybrids obtained via porphyrin adsorption on the clean SiC(110), as observed for other wide gap materials such as TiO2, although questions are still open on the role of surface defects and on the presence of different oxidation levels of the exposed substrate surfaces. Modification of the surface may, however, take advantage of grafting through pyrrole groups, as linker for further functionalization. [A. Catellani and A. Calzolari, J. Phys. Chem. C 116, 886 (2012)].

Our main concern in figuring out the potential of nanowires for optoelectronics is a quantitative comparison between the internal quantum efficiency of GaN-based nanowires and the internal quantum efficiency of equivalent planar heterostructures. In order to reach this goal, ordered arrays of GaN nanowires with axial or radial InGaN quantum wells have to be grown with an optimized protocol. The structural properties and the emission characteristics of single nanowires and ensembles have to be investigated in detail. In particular, also the influence of the surface needs to be analyzed. To avoid fluctuations in density and in dimension, which lead to significant dispersion in the optoelectronic properties of the nanowires ordered arrays are grown by selective area growth on a pre-structured substrate. In future perspective this bottom-up mode of fabrication allows a precise addressability of each component and can therefore be compatible with integrated device platforms. Pyramidal tips defined by semi-polar crystal facets characterize the GaN nanowires grown by molecular beam epitaxy in regular arrays on Ga-polar GaN pre-structured templates. The internal quantum efficiency is expected to increase for Gan/InGaN/GaN quantum wells grown on these semi-polar planes as compared to the common polar ones. Therefore their growth and optical characteriza-tion will be the next task to be addressed.

Potential applications of semiconductor nanowires include logic devices and the challenge of microelectronic device scaling has motivated research on semiconductor nanowires for nanoelectronics. Based on a quantum kinetic approach, an open source simulation tool (http://sourceforge.net/projects/nwfetlab) has been developed for the realistic numerical analysis of few-electron transport in nanowire-based field-effect transistors. The theoretical model is based on a non-equilibrium Green’s function technique. Few-electron Coulomb charging effects are taken into account with the help of a many-body multi-configurational approach. The influence of a coaxial gate electrode is considered by means of a Coulomb Green’s function. In turn, the simulated current-voltage characteristics are in very good qualitative agreement with known ex-perimental studies of nanowire transistors in the Coulomb-blockade regime.

Within the framework of nanoelectronics and for energy harvesting semiconductor nanowires are studied for increasing the efficiency of photovoltaic cells and, at a much smaller scale, for self-powering nanodevices, such as sensors, nanoelectronic devices or robots, by harvesting heat or mechanical energy waste from the environment. We have undertaken the study of the thermal conductivity of single SiC and core-shell SiC-SiO2 nanowires that can be measured by Raman scattering thermography. The thermoelectric properties are also studied by modelling the phonon-mediated heat transport in micro- and nanowires based on phonon hydrodynamic equations. This study will be complemented with additional simulations of the thermal properties of nanowires based on molecular dynamics.

Research on semiconductor nanowires these days provides an excellent platform for the training of a gen-eration that will be expected to play a leading role in developing new ideas and concepts for a truly new technology of the future.

Angela Rizzi (“nanowiring” coordinator)
www.nanowiring.eu rizzi@nanowiring.eu
Marie Curie Initial Training Networks, GA 265073